The object of the invention is improvements to methods for manufacturing hollow bodies, namely containers such as bottles, flasks, etc., obtained by blowing of plastic blanks in finishing molds; its object is also a device and an installation for implementing the method.
It can be applied in particular but not exclusively to the manufacture of small capacity hollow bodies, typically on the order of a half liter or less.
To manufacture a hollow body such as a container of a plastic such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN) or polyvinyl chloride (PVC) in a single layer or multiple layers, it is known to produce a plastic blank, then to place this blank while it is at a softening temperature in a finishing mold, and to inject a blowing fluid (typically air) in the blank to transform it into a recipient.
Thus, so-called extrusion-blowing methods are known in which the blanks, referred to as parisons, are extruded tubes. To produce a container, a parison is closed in a mold and air is injected into the parison.
Injection-blow molding methods are also known in which the blanks are preforms obtained by injecting the plastic into the mold; then, after they are produced, the preforms are either transferred to the finishing mold and then immediately transformed into containers in the finishing mold (methods known as hot-cycle) or stored or transported before being softened by heating and transformed into containers in the mold (methods referred to as cold cycle).
A disadvantage of known machines for manufacturing hollow bodies lies in the generation and transfer of the blowing fluid to the blanks.
Indeed, blowing requires high pressure levels (typically ranging from 10 to 20 bars in the case of extrusion-blowing and in the 40-bar range in the case of injection-blow molding), thus causing considerable consumption of fluid. Blowing a one-liter hollow body produced by extrusion-blowing requires 10 to 20 liters of fluid; blowing a one-liter hollow body produced by injection-blow molding requires 40 liters of fluid.
To obtain sufficient pressure and flow levels with respect to production rates (1,200 containers per hour and per mold in the applicant""s injection-blow molding machines), known machines are connected with at least one compressor that supplies the high pressure necessary for blowing.
The compressor operates continuously to be able to supply the quantity of air required for all the molds comprising the installation. As for the blowing, it takes place sequentially.
The primary disadvantages of such a structure are as follows:
first of all, due to the continuous operation of the compressor whereas blowing takes place sequentially, electrically or mechanically controlled valves must be provided to allow or prohibit the insertion of blowing fluid in the molds. These valves are continuously subjected upstream to the high pressure levels required for blowing. They therefore must be resistant and have a costly, complex design. Due to the operating speeds to which they are subjected, they deteriorate quickly. They thus constitute consumables that should be replaced relatively often:
the compressors used are accessory devices for the installations. Besides their cost and burdensomeness, it is necessary to provide fluid connections with the installations. These connections increase the risks of malfunction (leaks in the event of disconnection);
in addition, the same compressor is generally used to supply several installations. If this device fails, this may result in considerable production losses, since several installations may be dependent upon a single compressor;
with installations using a rotating carrousel technology in which the molds are carried by the carrousel, it is necessary to provide a rotating fluid connection to carry the high-pressure blowing fluid to the molds. In this case as well, it is a matter of a highly sensitive part with very restrictive manufacturing tolerances.
The objective of the invention is to remedy these disadvantages.
According to the invention, a method for manufacturing a hollow body by blowing a blank into a finishing mold with the help of a blowing fluid, is characterized in that it consists of connecting to each mold a separate compression chamber formed by a cylinder-piston assembly; establishing an initial fluid pressure in the chamber when it is at its maximum volume; reducing the chamber volume to compress the fluid while keeping the chamber and the inside of the blank isolated; connecting the chamber and the blank when the chamber volume reaches a defined value in order to initiate blowing of the blank by retention of the compressed fluid, and continuing to reduce chamber volume to a minimum while maintaining the connection with the blank and ending the blowing by compressing the fluid volume remaining in the chamber and transferring to the blank.
By connecting to each mold a compression chamber inside which the pressure ranges from a minimum (initial pressure) to the blowing pressure, there is no longer a device continuously undergoing the latter pressure.
In a form of construction, a controlled valve is arranged in the circuit between the chamber and the mold, but this valve undergoes pressure rise and drop cycles upstream. It is thus less restrictively subjected to stress.
The invention reduces the length of the fluid connections since the chamber can be placed as closely as possible to the corresponding mold.
An installation is no longer dependent upon a compressor. As a result, if a chamber is defective, it is possible to keep the rest of the installation running at least provisionally. In addition, two installations are not dependent on each other.
Lastly, high-pressure rotating fluid connections are no longer necessary even when the molds are carried by a carrousel, since each mold is connected with its respective chamber.
Another advantage of the invention is that compression of the air causes its temperature to rise, which considerably promotes blowing when the blank and thus the hollow body is of a thermoplastic matter. Indeed, if the air temperature exceeds the material""s softening temperature, it prevents the material from solidifying during blowing.
The blowing temperature depends in large part on the initial pressure and, of course, on the compression ratio. It also depends on the predetermined piston position at which blowing is initiated.
This is why the initial pressure established in the compression chamber is preferably greater than the ambient pressure and, according to another characteristic, is established at least in part by an external low-pressure source.
Low-pressure source refers to an industrial source currently present in companies, ranging from 1 to 15 bars and typically 7 bars, for example.
Because it is low, there are no connection problems like those with a high-pressure source, as used to occur with installations of the prior art. Leakage risks are limited and the technology of fixed as well as rotating connections is perfectly mastered and much simpler.
Thus, for example, with a compression chamber with an initial volume of 1.5 liters, and initial pressure of 7 bars, an item of 300 cc is inflated at 35 bars (by initiating the connection between the chamber and the blank, thus the mold when the piston position is such that chamber volume reaches 300 cc).
With the same chamber, only 5 bars would be attained if the initial pressure were ambient pressure. To attain 35 bars, and blow a 300-cc article starting with ambient pressure, the chamber would have to have an initial volume of 7xc3x971.5 liters, i.e., 10.5 liters.
The installation for implementation would become somewhat cumbersome.
According to another characteristic, initial pressure is obtained at least in part by returning the high-pressure fluid contained in the hollow body to the chamber when it is degassed.
According to another characteristic, the low-pressure circuit is connected so as to only supply fluid when the blowing cycle is established. It brings all of the fluid only when no hollow body has been blown, or when a blank has burst during blowing.
Indeed, after blowing, the total fluid circuit volume has increased by a level corresponding to the difference between the final volume of the hollow body and the initial volume of the blank. Not counting the initial volume of the blank and that of the connections between the chamber and the mold, and by taking the same parameters as previously, total fluid circuit volume would be 1,500 cc+300 cc after blowing, i.e., 1,800 cc filled with 10.5 liters of air, making it possible to obtain a 5.8-bar residualxe2x80x94thus initialxe2x80x94pressure for the subsequent blowing in the chamber.
Thus, the external source should only bring the supplement in order to obtain the 7 bars necessary in the case in question.
In this way, one achieves savings on the order of 80% in terms of external fluid brought in.
In this respect, the method is self-regulating: as indicated, if a blank accidentally bursts during blowing, the external supply will be total at the subsequent blowing.
According to another characteristic, a device for implementing the method comprises a compression chamber connected to a mold and consisting of a cylinder in which a piston is arranged; a fluid circuit connecting the compression chamber with means to establish an initial pressure in the compression chamber; means to connect the compression chamber and the inside of a blank placed in the mold when the piston position attains a predetermined position, and in that the length of the cylinder is such that after connecting, the piston""s stroke continues to complete the blowing by transferring the fluid volume remaining in the chamber to the blank.
According to another characteristic, an installation comprising at least one device for blowing a hollow body also comprises: a chassis as well as a structure rotating around a pivot borne by the installation""s chassis; at least one mold is attached to this rotating structure and is connected to its respective device; a first tip of each cylinder-piston assembly is connected to a respective first axle borne by the chassis, parallel to the pivot""s axis and at a distance from it that defines the desired piston stroke; a second tip of each assembly is connected to a respective second axle borne by the rotating structure toward a peripheral zone thereof.
Thus, due to the eccentricity between the first axle and the pivot, an alternate movement of the piston is caused relative to the cylinder when the rotating structure rotates. In actual fact, the distance between the pivot and the first axle corresponds to half of the piston stroke.
In a preferred form of construction, the piston control stem is connected to the first axle, and the cylinder is connected to the second axle.
According to another characteristic, the installation comprises at least two molds and thus the same number of piston-cylinder assemblies; the first axle is shared by each of the assemblies and the second axles are arranged on the rotating structure at different positions equidistant from the pivot.
Each piston thus carries out the same movement as the other ones with a phase shift.
In these cases, the second axles are preferably spread angularly regularly on the rotating structure.
The cycle is thus regular.